Module 1: Incidence and prevalence

Question 1

PECOT

Answer 1

• Population: group of people who share a
  specified common factor.
• Exposure group
• Comparison group
• Outcomes:
  • EGO: occurrence of dis-ease in exposure
  group.
  • CGO: occurrence of dis-ease in comparison
  group.
• An average can be taken and EG
  compared to CG.

Question 2

Incidence

Answer 2

• Incidence: counting the number of onsets of disease events occurring during a period of time.
  • Longitudinal measure producing a rate.
  • Most appropriate for observable events.
  • Require dis-ease outcome to be categorical
  variable.
  • Measuring prevalence at two points of time
  and calculating the change in prevalence bt/
  the two points in time is a measure of the
  incidence of dis-ease over the period bt/ the
  two time points

ADVANTAGES
- Most useful for measuring causes of dis-ease
occurrence.
- Incidence is determined only by the dis-ease
risk in the population.
- Measures of incidence include events,
population and time.

DISADVANTAGES
- Incidence can be difficult to measure as one has
to observe events over time.

Question 3

Prevalence

Answer 3

Prevalence: counting the number of people w/ a
dis-ease at a point in time.
• Cross sectional measure producing a figure.
• Most appropriate when transition from a nondis-eased state to a dis-eased state cannot
easily be observed and counted.

ADVANTAGES
- Prevalence is relatively easy to measure, as it is
static, taken for one point in time.
- Useful to funders and planners of health.

DISADVANTAGES
- Prevalence measures only include events and
population.
- Prevalence is determined by incidence, death
rate and cure rate

Question 4

Prevalence

Answer 4

POINT PREVALENCE
- Point prevalence: outcome does not take any
previous time period into account and is simply
measured at one point in time.

PERIOD PREVALENCE
- Period prevalence: outcome/numerator depends
on the time period specified.
• Dis-ease outcomes cannot easily be
measured at one point in time, so we look
back and measure them over a period of
time.

A population could have a high incidence/low
prevalence if death/cure rate is also high.
A population could have a low incidence/high
prevalence if death/cure rate is also low.

Question 5

C o m p a r i s o n s — R i s k

Differences and Relative Risk

Answer 5

• Differences bt/ EGO and CGO can provide
  insight into the size of the effect of the study
  exposure on the dis-ease outcome.
• Comparisons of dis-ease occurrence typically
  called ‘estimates of effect/association’ of an
  exposure on a dis-ease outcome.

Question 6

RISK DIFFERENCE

Answer 6

• EGO-CGO
• Units — same unit as EGO/CGO calculation e.g.
  per x people over y years — more info.
• Risk ratio can be the same whereas risk
  difference can be much smaller/larger.
• Also called difference in occurrences, absolute
  risk (difference).
• RD=0, no difference in effect of E and C on the
  study outcome.
• Risk difference is an absolute risk reduction if
  risk is lower in the exposure group or an
  absolute risk increase if the risk is higher in the
  exposure group.

Question 7

RELATIVE RISK

Answer 7

• EGO/CGO
• Also called risk ratio, relative risk difference,
  ratio of occurrence.
• No units — less information.
• RR=1, ‘no-effect’ value.
• If dis-ease occurrence measures are calculated
  as averages, relative comparison of two mean
  scores is ‘relative mean’ (RM).
• Relative risk reduction: relative risk is subtracted
  from 1.0, then expressed as a percentage.
• Relative risk increase: 1.0 subtracted from
  relative risk, then express as percentage.

Question 8

Non Random Error

Answer 8

• Also called biases/systematic errors.
• If error occurs because of poor study design,
  processes or measurement.
  • Valid study: small amount of random/nonrandom error.

Question 9

RECRUITMENT -RAMBOMAN

Answer 9

• Are participants representative sample from a
  defined population?
• Described as external validity error, as when
  present findings may not be applicable to wider
  population.
• Particularly important when major objective of
  study to measure characteristics of real population but participants recruited not
  representative of eligibles.

Selection bias: participants allocated to EG
different source to participants allocated to CG.
• Confounding error caused by allocation
process.
- Non response bias: non-responders different to
responders. Consider response rate.

Question 10

ALLOCATION

Answer 10

How well were participants allocated to EG and
CG?
- Confounders: EG and CG differ in ways other
than allocation, and these other difference have
an effect on the study outcome.

• RCTs: allocation by random process; all
  participants have equal chance of allocation to
  EG or CG, so groups are similar.
  • RCTs are known as experiments, because
  investigators actively control allocation
  process.
  • RCTs best way to stop confounding.
• Complete baseline comparison to ensure
  RCTs w/ small samples don’t have different
  groups just through chance alone

 Concealment of allocation stops tampering
w/ randomisation process.
- Observational studies: allocate by measurement
and assign to EG and CG accordingly.
• People may lie/under-report to hide
embarrassment or they can’t remember.
- Avoid by good questionnaire design.
• Inaccurate measurement of exposure:
(allocation) measurement error.
• CG and EG often quite different: confounders
if study outcome influenced.
- Adjust for in analyses.
- Sufficient information must be collected
about other differences for adjustment
purposes.
- Confounding present in almost every
observational study.
• Two or more effects mix, all

Question 11

MAINTENANCE

Answer 11

Will the validity of the study results be affected
by how well they were maintained in EG and
CG?
- Maintenance error: some participants’ exposure
status changes, or some are lost to follow-up.

• Did participants remain in their allocated
groups? Did they maintain their initial
exposure/comparison exposure?
- Long term cohort studies prone to maintenance
bias — offset by regular follow-ups.
- Maintenance not a problem for cross-sectional
studies.

Question 12

BLIND AND OBJECTIVE MEASUREMENT

Answer 12

• Reduce error by blinding participants and
investigators to knowledge of which
intervention participants received.
• Blinding of outcome measurement ideal for
death certification when pathologist blind.
• R e d u c e e r r o r b y t a k i n g o b j e c t i v e
measurements where possible e.g. using a
machine.
- Measurement: use a standard definition.

Question 13

ANALYSES

Answer 13

• Confounding can be reduced by dividing
  participants into strata — stratified analysis e.g.
  age standardisation.

Question 14

Random Error

Answer 14

• Due to chance.
• No single study will ever measure the exact truth
  in the whole population, even if it is a perfect
  study.
  • Every study an ‘estimate of the truth’.
  • Identical studies will produce different results.
• All measures of EGO/CGO/RR/RD/NNT have
  random error.
  • Most random errors can be reduced by
  i n c re a s i n g s a m p l e s i z e , re p e a t i n g
  measurements.
• Extreme events are often chance events:
  repeating measurements or studies w/ extreme
  results many times usually gives less extreme
  results — regression to the mean.

Question 15

RANDOM SAMPLING ERROR

Answer 15

• Inherent in every study.
  • N o s a m p l e w i l l e v e r b e p e r f e c t l y
  representative of the population.
• Every sample will differ due to chance, and
  never include participants w/ identical
  characteristics.
• Bigger sample, smaller differences bt/ sample
  and population

Question 16

RANDOM MEASUREMENT/ASSESSMENT ERROR

Answer 16

• R a n d o m m e a s u r e m e n t e r r o r e ff e c t s
  measurement of both exposures and outcomes.
• Ability to measure biological factors the same
  way every time is often poor, particularly when a
  human operator is involved.
  • Avoid by repeated measurements w/ an
  average, or use, automatic, objective
  machine.
• Randomness inherent in biological phenomena.
  • Inherent variability in biological phenomena,
  and therefore in its measurement.
  • Identical measurements of exposures and
  outcomes in the same or similar people can
  change from moment to moment.

Question 17

RANDOM ALLOCATION ERROR

Answer 17

• Groups in an RCT may differ by chance alone,
  particularly if trial is small.
• Reduce by undertaking a larger study.

Question 18

CONFIDENCE INTERVALS

Answer 18

• Every epidemiological measure has random
  error in the estimate of the truth (EGO, CGO, RR,
  RD) in the population that the study participants
  were recruited from which can be estimated by a
  confidence interval.
• 95% CI definition: in 100 identical studies using
  samples from the same population, 95/100 of
  the 95% CIs will include the true value for the
  population.
  • There is about a 95% chance that the true
  value in the population from which the
  participants were recruited lies within the
  95% CI, assuming no random error.
  • Bigger the study, narrower the CI.
• As number of events in study increases, recruited. Probably no statistically significant difference
  • 95% CIs for RR and RD usually cross no effect line.
  • Study results not statistically significant.
  • If 95% CIs of RR and RD cross no effect line,
  best stated as too much random error to
  determine if there is a difference bt/ EGO and
  CGO, as opposed to stating there is no
  statistically significant difference.
• 95% CI just touches no-effect line, study
  “borderline statistically significant” (compare w/
  statistically/not statistically significant).
• Statistically significant event may be clinically
  significant if a clinician would make a similar
  decision whether the true result was near one
  end of the confidence interval or the other.
  Similarly, a small but statistically significant
  effect w/ a narrow confidence interval may not
  be of clinical significance.
  width of 95% CIs decrease.
  • Wider the interval, more random error in
  measure.
  • 95% CI most common for epidemiological
  studies.
  • 99% CI wider than a 95% CI.
  • Confidence limit: each end of CI.
• If no overlap of confidence intervals, reasonable
  to assume that EGO and CGO are truly different
  from each other in the underlying population.
  • When there is no overlap of CIs in EGO and
  CGO, confidence intervals for RD and RR will
  not cross no-effect line.
  • Therefore measures of association bt/ EGO
  and CGO show real effect and ‘study results
  are statistically significant’.
• If overlap of CIs, study unable to determine if
  EGO is different from CGO in the population
  from which the study participants were

Question 19

META ANALYSES

Answer 19

Meta-analysis: analytical technique use to
combine results of studies mathematically to
generate a summary estimate of the effect. The
result could be statistically significant.
- Next best thing to a large study and reduces
random error in effect estimates.
• Does not reduce non-random error.
- Most studies too small to produce precise
estimates (i.e. w/ low random error) of disease
frequency therefore measures of association
also generally imprecise.
- Point estimates for different studies all show a
RR difference, but the 95% CIs cross the no
effect line.
• Conventionally, none of the trials are
statistically significant.
• Does not necessarily mean treatment not
beneficial, as studies could simply have been
too small.
• Studies must have low levels of bias (nonrandom error) and reasonably similar findings.
• Combining multiple similar studies in a metaanalysis is an alternative to conducting one
large study.
- Meta-analyses commonly undertaken to
combine results of multiple RCTs which
individually have too much random error to
demonstrate whether or not intervention
has a real effect.
• Ideal meta-analyses combine studies from
systematic reviews, however meta-analyses
can be taken of a non-systematically
recruited group of studies, although one
should be wary of the latter.

Answer 20

Answer 21

Question 22

Dahlgren and Whitehead and Causes of the causes

Answer 22

DETERMINANTS FOR INDIVIDUALS
- Any event, characteristic or other definable entity that brings about a change for better or worse
in health.
• Age, sex, ethnicity
• Water, shelter, sanitation (basic survival)
• Income, occupation, employment, education (provides opportunities for housing etc.)
• Housing, housing tenure, overcrowding, neighbourhoods (design of urban environments, is
one supportive and supported?, social and built environments)
• Access
• Deprivation
• Societal characteristics e.g. racism, attitudes to alcohol or violence, value on children (social
norms)
• Unconscious bias — racism, bullying, harassment
• Autonomy and empowerment — social cohesion (attitudes, resilience)
- What causes people to engage in activity which is a risk/protective factor — determinant of
determinant.

Question 23

DETERMINANTS AND THE LIFE COURSE

Answer 23

• Determinants of health may vary at different life stages.
• Three ways in which life course events can interact to influence long-term health and wellbeing:
  • Cumulative — notion of poverty trap, born into more or less wealth, children of parents of
  grandparents.
  • Multiplicative — multiplied effects on risk— multiple CVD risk factors combine to give
  fractional increase.
  • Programming — Barker hypothesis — genetics, in utero impacts critical periods in one’s
  development.

Question 24

DAHLGREN AND WHITEHEAD MODEL

Answer 24

Person — micro (individual)
- Non-modifiable genes and hereditary,
and individual lifestyle factors, e.g.
attitudes to smoking, alcohol, diet,
exercise, influenced by social and
community networks.

• Community — meso (family, living, work)
- Local influences such as home, workplace and neighbourhood (behaviour of family,
friends, groups like me, community and its makeup.), and wider societal levels such as
health and education.

• Environment — macro (national/global)
- Cultural, social, political, physical and built
environment.
- Shared beliefs, values, attitudes and culture of broad
groupings.
- Govt regulation and policy.

Question 25

Upstream and downstream

Answer 25

UPSTREAM VS DOWNSTREAM INTERVENTIONS
- Downstream interventions operate at a micro (proximal) level, including treatments systems,
and disease management.
- Upstream interventions operate at the macro (distal) level, such as government policies and
international trade agreements.

Question 26

PROXIMAL AND DISTAL DETERMINANTS

Answer 26

Proximal determinants of health near (directly and readily associated e.g. lifestyle and
behavioural factors) the change in health status.
- Distal determinants of health are either distant in time or place from the change in health status.
• Also referred to as upstream factors, e.g. national, political, legal and cultural factors that
indirectly influence health by acting on proximal factors.

Question 27

STRUCTURE AND AGENCY

Answer 27

• Structure: social and physical environmental conditions/patterns (social determinants) which
  influence choices and available opportunities — upstream (three outer arches).
• Agency (empowerment): capacity of an individual to act independently and make free choices
  — downstream (two inner arches).

Question 28

Māori health: HISTORY

Answer 28

• Early contact
  • Māori initially flourished economically and
  socially.
  • Beginning of complex changes.
• Official engagement
  • Colonisation, Declaration of Independence,
  Treaty of Waitangi, New Zealand.
  • Herald of an era of depopulation, disease and
  dispossession.
• Colonisation
  • Not value-free.
  • Assumptions held by colonisers.
  • Notions of superior and inferior peoples.
  • Notions of civilisation especially religious but
  also economic and scientific e.g. land use,
  conservation.
  • Notions of deserving and undeserving.
• Societal barriers still obvious today.

Question 29

IMPLICATIONS OF TREATY

Answer 29

Creation of Government — Article I
• Construction of state sector — justice system,
education, health, welfare etc.
• Constitution Act 1852 created settler
Government, determined voting rights
- Laws and policies — Article II
• Disregard for Māori voice/authority despite
Article II
- Māori land — historical basis of settler wealth
• Pre-emption clause of Treaty
- Māori land court — 1860s
- Individual title
- Different or denied citizenship — Article III
• Old-age pensions 1898
- Equal provisions for Māori and pakeha.
- Asian excluded.
- Māori access difficult — through Māori land
court.
- Māori regularly removed from rolls.
- Reduced amount paid to Māori.
• Social Security Act 1938
- Underpayment continued after WWII.

Question 30

RELATIONSHIP OF TREATY TO HEALTH

Answer 30

• Policy alienation.
• Land alienation:
  • Social disruption of community.
  • Breakdown of political power and alliances.
  • Economic resource depletion and poverty.
  • Resentment by indigenous peoples.
• Unequal (inferior) citizenship:
  • Entrenchment of poverty and dependance.
  • Increased barriers to development.
  • Acceptance of inequity by non-indigenous
  groups.
  • Resentment, frustration and anger.
  • Social breakdown, crime, high risk behaviours.

Question 31

Prevention, promotion, protection: approaches to

taking action

Answer 31

Population-based (mass) strategy
- Whole population.
- Reduce health-risks/improve outcome for all individuals of
population.
- Useful for common disease/widespread cause.
• Immunisations, seatbelts, low salt foods in supermarkets.
ADVANTAGES
- Radical, addressing underlying causes.
- Large potential benefit for whole population.
- Behaviourally appropriate — change social norms; something becomes acceptable.
DISADVANTAGES
- Small individual benefit.
- Poor individual motivation.
- Whole population exposed to downside of strategy (less favourable benefit to risk ratio).

Question 32

High-risk individual strategy

Answer 32

• Focuses on perceived high-risk individuals.
• Well matched to individuals and their concerns.
  • Obese adult interventions, intravenous drug users (NZ Needle Exchange), high systolic BP
  individuals.

ADVANTAGES

• Tailored and targeted; appropriate to individuals.
• Individual motivation.
• Cost-effective use of resources.
• Favorable benefit to risk ratio.

DISADVANTAGES

• Cost of screening; need to identify individuals, and
• Temporary effect (e.g. breast screening cohort).
• Limited potential (small sample size).
• Behaviourally (culturally) inappropriate.

Question 33

Health Promotion

Answer 33

Acts on determinants of wellbeing.
- Positive, health/wellbeing focus.
- Enables/empowers people to increase control over
— and improve — their health.
- Involves whole population in everyday contexts.
- Primary-care orientated.

Question 34

OTTAWA CHARTER FOR HEALTH PROMOTION (WHO

Answer 34

21 November 1986.
- First International Conference on Health Promotion.
- ‘Mobilise action for community development’.
- Health is:
• A fundamental right for everybody.
• Requires individual and collective responsibility.
• Opportunity to have good health should be equally available.
• Good health is an essential element of social and economic
development.
- Three basic strategies:
• Enable — individual
- Provide opportunities for all individuals to make healthy choices through access to
information, life skills and supportive environments.
• Advocate — systems
- Create favourable political, economic, social, cultural and physical environments by
promoting/advocating for health and focusing on achieving equity in health.
• Mediate — individuals, groups and systems
- Facilitate/bring together individuals, groups and parties w/ opposing interests to work
together and compromise for health promotion.
- Five priority action areas:
• Develop personal skills: life skills education, ICT for health, awareness campaigns.
• Strengthen community action (community empowerment): Self-help groups and community
organised services (e.g. HIV/AIDS), initiatives to promote healthy schools, cities etc.
• Create supportive environments: air control, water and sanitation, speed bumps, work safety.
• Reorient health services towards primary health care: care responsive to needs of patients/
families (considering culture, social norms, physical and mental capacities, healthcare skills,
aspirations, resources), healthcare education, amenities to enhance hospital experience.
• Build healthy public policy: Tax alcohol and cigarettes, seat-belt use, ban smoking in public
places, food and drug control, mandatory sports in schoo

Question 35

Disease Prevention

Answer 35

Disease focus.
- Looks at disease/injury and ways of preventing them.
- Primary: limit incidence by controlling specific causes and
risk factors e.g. seatbelts, immunisation.
- Secondary: reduce more serious consequences of disease e.g. hip fracture screening, lifeguard
services.
- Tertiary: reduce progress of complications of established disease: counselling services for
PTSD, rehabilitation for burns patients.

Question 36

Health Protection

Answer 36

Predominantly environmental hazard focused.
- Risk/hazard assessment e.g. environmental
epidemiology, biosecurity, safe air and water.
- Monitoring e.g. biomarkers of exposure to
hazardous substances.
- Risk communication e.g. relating environmental
risks to public.
- Occupational health e.g. workplace safety
regulations.

Question 37

Establishing causality in

population health

Answer 37

Association and causation
- Explore the concepts of association and causation in
understanding the determinants of health and
attempts to improve population health.

EPIDEMIOLOGY AND THE CAUSES OF DISEASE
- Epidemiology, causes of disease, appropriate
preventative measures can be introduced.
- Not determine disease cause in individual, but
relationship/association bt/ exposure—disease in
populations.
• Links exposures—outcomes.
• Lind’s controlled-trial of 12 sailors (small sample
size) showed citrus consumption associated w/
scurvy prevention.
- Preventative action taken before the cause
identified

Question 38

Bradford Hill and Rothman’s Criteria

Answer 38

BRADFORD HILL CRITERIA (1965)
- Establish causality. - Aid, not definitive checklist.
- Not all criteria needed to establish causality.
• Temporality.
- Essential.
- First cause then disease.
• Questionnaire to establish exposures then
deaths followed up.
• Strength of association.
- Stronger association, causality more likely, in
absence of known biases (selection,
information, confounding).
• Consistency of association.
- Replication of findings by different investigators,
different times, places, different methods.
• Biological gradient (dose-response).
- Incremental change in disease rates—
corresponding changes in exposure.
• Biological plausibility of association.
- Does the association make sense biologically?
• Specificity of association.
- Weakest criteria. - A cause leads to a single effect, although a
single cause often leads to multiple effects.
• Reversibility.
- Demonstration that under controlled conditions,
changing exposure causes a change in the
outcome.

Question 39

ROTHMAN’S CAUSAL PIES

Answer 39

• A cause of a disease: event, condition, characteristic
  (or combination) playing essential role in producing
  disease.
  • Sufficient cause (causal mechanism): factor(s)
  inevitably producing specific disease.
  • Component cause: factor contributing to disease
  causation, not sufficient alone to cause disease.
  • Necessary cause: factor (component cause) must
  be present for disease to occur.
• Use association/other factors to infer causation and
  intervene to prevent disease.
• Can intervene at any point of pie.
• Knowledge of complete pathway not prerequisite for
  introducing preventative measures.
• Removing one of component causes can prevent
  disease.

Question 40

Screening: a special type
of prevention strategy
S u c c e s s f u l s c r e e n i n g
initiatives

Answer 40

SCREENING FUNDAMENTALS
- Screening: identifying risk factors for
disease/unrecognised disease by applying
tests, large scale to population.
- Can be primary (alcohol intake to prevent
breast cancer)/secondary (breast cancer
screening)/tertiary (screening for bone
density after breast cancer chemotherapy).

Question 41

SCREENING COMPONENTS

Answer 41

COMPONENTS
- Eligible population (most susceptible to
disease).
- Screening test: simple, cheap, easily
applied to many.
- Diagnostic test: best possible test for
testing disease presence.
- Intervention/treatment.
- Re-screening at given time interval.

Question 42

Diseases and screening tests

Answer 42

SCREENING CRITERIA
- Suitable disease:
• Important public health problem:
relatively common/relatively uncommon
but early detection and intervention leads
to better outcome.
• Knowledge natural history disease (or
relationship risk factors—condition).
- Detectable early (detectable risk factor/
disease marker).
- Increased pre-clinical phase duration
— screening more effective: more time
to screen, treat.
- Suitable test:
• Reliable (consistent results), safe, simple,
affordable, acceptable, accurate (test’s
ability, indicate which individuals have
disease/not).
- Accuracy: gold standard diagnostic
test, but can’t be applied to large
populations. Screening test judged
against diagnostic test, less expensive.
- Suitable treatment:
• Evidence early treatment->better
outcomes.
• Effective, acceptable, accessible
treatment.
• Evidence-based policies covering
eligibles for treatment/appropriate
treatment to be offered.
- Suitable screening programme:
• Benefits must outweigh harm.
• RCT evidence screening programme will
result in:
- Reduced mortality.
- Increased survival time.
• Lead time bias.
• Length time bias.

Question 43

Screening test performance

Answer 43

DEFINITIONS

• True positive: disease, positive test.
• False negative: disease, negative test.
• False positive: no disease, positive test.
• True negative: no disease, negative test.

Question 44

INTERPRETATIONS OF SENSITIVITY AND

SPECIFICITY

Answer 44

• Sensitivity high if proportion of true
positives high.
• Specificity high is proportion of true
negatives is high.
- Fixed characteristics of tests.
• In two equally sized populations w/
different prevalences of disease,
predictive values will be different.

Question 45

Prioritizing in public health prioritization

Answer 45

EVIDENCE-BASED MEASURES
- Descriptive:
• Consider current problem (outcomes),
who most/least affected (e.g. Treaty
obligations may prioritise one disease
over another, e.g. even though stroke
overall greater death rate, respiratory
disease rates greater for Māori

- Explanatory:
• What are determinants/risks? Why
getting worse/better? Why populations
different? Epidemiological measures used
in prioritisation e.g. determine major risk
factors of disease burden.

- Evaluative:
• What can improve health outcomes and
in whom? How well can problem be
solved?
- Target population, expected number in
population reached, evidence of
effectiveness (based on known
success rates), cost.

- Age at death/premature mortality —
years of potential life lost to death
(YLL).
- Time lived w/ disability — years lived
w/ disability (YLD).
- Population Attributable Risk.
• D o e s p r o b l e m / r i s k f a c t o r
disproportionately affect population sub
groups? Why? ToW considerations. 

• Economic feasibility: economic sense to
address problem/consequences of not
doing so?
• Opportunity cost: potential other health
benefits had money been spent on next
best alternative intervention/healthcare
programme.
• Acceptability: Will community/target
population accept problem being
addressed? Competing interests?

Question 46

Population Attributable Risk

Answer 46

ATTRIBUTABLE RISK
- RD=AR (EGO-CGO).
• EGO = incidence in exposed population.
- Amount of extra disease attributable to
particular risk factor in exposed group.

POPULATION ATTRIBUTABLE RISK
- Amount of extra disease attributable to
particular risk factor in particular population.
- If association causal, amount of disease
theoretically preventable if particular risk
factor removed from population.
- RR: Compared w/ non-smoking mothers,
smoking mothers are four times more likely
to have a pre-term baby

• RD: Smoking mothers will have 600 more
  preterm babies per 1000 women than nonsmoking mothers.
• PAR: For every 1000 mothers in population,
  we can prevent 90 preterm births by
  removing heroin addiction as a risk factor.
• PAR higher when disease prevalence higher
  in population.